Shading Gold Leaf with Asphaltum
Many of the old timers in the sign industry have used a transparent brown liquid, called asphaltum, to shade their gold leaf work. Although this oily substance smells just like tar, the two materials are definitely not the same. While tar is a by-product of the distillation of coal, one type of asphaltum, is a by-product of oil refining. Gilsonite asphaltum is another type of asphaltum, which is found in nature.
To a great extent, the transparency and ease of use of asphaltum accounts for its popularity as a glaze in the sign industry. There’s just one problem. For exterior sign applications, asphaltum just doesn’t stand the test of time.
Exposed to extreme changes in temperature, asphaltum tends to crack. When used to shade window lettering, over time asphaltum can also fade when subjected to ultra-violet light. To extend its durability, some sign makers will combine asphaltum, Japan paint (such as burnt sienna or burnt umber) and a varnish (such as Spar Varnish), and then thin the mixture with turpentine. This combination seems to greatly improve the longevity of the shading.
When using asphaltum to shade surface gilding you can brush on the glaze and then remove the excess with a lint-free cotton rag. After shading, a coating of varnish provides additional protection. For second surface gilding on glass, I have gradually built up the density of the shading, as shown in the photo above of the foliage and scroll.